1. Field of the Invention
The present invention relates to a telephoto-type zoom lens system and, more particularly, to a telephoto-type zoom lens system free from aggravation of aberrations for the whole range of photographing distances from the infinite distance to short distance.
2. Description of the Prior Art
Some of known zoom lens systems in general are arranged to be focused by the first lens group. That is, the first lens group is advanced for focusing according to the photographing distance. This kind of focusing method, however, has a disadvantage that aberrations are aggravated considerably when the photographing distance changes as in the case of the front-lens group advancing method adopted for low-cost cameras. If, however, the lens system as a whole is advanced for focusing, in case of zoom lens systems, it is impossible to keep the focusing position constant in the zooming range. Therefore, the focusing method to advance the lens system as a whole cannot be adopted for zoom lens systems, and the focusing method to advance the front lens group is generally adopted.
Besides, the shortest possible photographing distance for zoom lens systems is usually decided at a constant distance for the whole zooming range. Therefore, even when the magnification for photographing on the short focal length side is small, the magnification for photographing on the long focal length side becomes large and, consequently, aggravation of aberrations on the long focal length side becomes necessarily large. Especially in telephoto-type zoom lens systems, aggravation of spherical aberration is conspicuous and, as a result, decrease of quality of the image and change of the focusing position are caused in the zooming range.
As a method to solve the above-mentioned problem in focusing of zoom lens systems, it may be considered to satisfactorily correct aberrations within the first lens group. In that case, however, it is necessary to adopt a first lens group of complicated arrangement and, as a result, the overall length of the lens system necessarily becomes long, weight of the lens system increases and, moreover, the lens system becomes extremely expensive. Therefore, this method is not practical for zoom lens systems for still-photographic cameras.
It may be also considered to solve the above-mentioned problem by dividing the first lens group into two lens components and to thereby correct aberrations in cases of close-up photographing. This method, however, is not preferable in practice to zoom lens systems for still-photographic cameras due to reasons same as the case of the above-mentioned method.
The most preferable method for solving the above-mentioned problem may be to arrange the first lens group by two or three lenses and to thereby reduce aggravation of aberrations in close-up photographing. Zoom lens systems of this kind of arrangement has excellent performance for photographing an object at the infinite distance. In close-up photographing, however, quality of the image decreases and, in addition, change of the focusing position in the zooming range becomes large. These disadvantages are caused by the fact that it is substantially impossible to favourably correct aberrations of the first lens group when a lens group of simple arrangement is adopted as the first lens group.